Introduction to metabolism
SCBCH2002
Humans, and many animals, eat erratically
• yet bodies must function efficiently and consistently
• nutrients are stored when supply exceeds demand
• stores are mobilised when required
– primary metabolic fuels are glucose and fatty acids
• diets may not be balanced
– ability to interconvert
metabolites is needed
• circulating [glucose] must be
maintained
– fuels neural tissues
Metabolism
• anabolism + catabolism
• metabolic conversions catalysed by
enzymes
• subject to complex controls
– hormones
– substrate availability
– need for product
Objectives
• explain the need for control of metabolic pathways within
a cell
• describe the major mechanisms for intracellular control of
enzyme activity
• explain the importance of allosteric control to normal
metabolic function
Enzymes
• are true catalysts
– increase reaction rate
– don’t shift equilibrium position
• may require cofactors
• are affected by pH and temperature
• substrates bind at active site
• may be activated/ inhibited
– by specific ligand binding
– by covalent modification
Feedforward activation
• process by which the activity of an enzyme catalysing a
reaction early in a reaction sequence is increased.
• activators may be metabolites which indicate that “raw
materials” are in plentiful supply, or ATP which indicates
the availability of energy for biosynthetic reactions
Feedback inhibition
• end product of a series of reactions inhibits activity of an
enzyme which catalyses one of the first steps
• prevents wasteful synthesis of compounds which are not
required
• end product of the reaction series may bind to one or
more of the subunits of an allosteric enzyme
Review Qs.... Part 1
1. Briefly define or describe
a. Feedforward activation
b. Feedback inhibition
Why do we need both forms of control?
Monomeric enzymes
• Km = substrate concentration at ½ Vmax
• typically mM - µM range
• thought to indicate intracellular [substrate]
• if an enzyme has multiple substrates:
– substrate with lowest Km has highest affinity for enzyme
•
activators and inhibitors may affect Km
– shift into (or out of) physiological [S] range
Glucokinase and hexokinase
• different enzymes catalysing the same reaction
– both glucose phosphorylating enzymes
• same activity, but very different Km
• glucokinase in liver
–
–
–
Km higher than normal BGL range
traps glucose efficiently when BGL high
quickly removes circulating glucose
after meals
• hexokinase in brain
–
–
very low Km
traps glucose very efficiently even
when BGL is low
Alcohol dehydrogenases
• Human alcohol dehydrogenases (ADHs) catalyze the rate-limiting step
in metabolism of various alcohols
• Km values vary:
ethanol
0.12-57 mM
methanol
ethylene glycol
isopropanol
2.0-3500 mM
4.3-2600mM
0.73-3400 mM
• Ethanol is the preferred substrate for ADH
Competitive inhibitors
• bind to the active site of the enzyme
• inhibitor competes with substrate for the active site
• reduce number of enzyme molecules available
E + S + I → ES or EI
ES → products
EI
↛ products
Competitive inhibitors may be
useful drugs
• can block reactions with negative outcomes
– cyclooxygenase inhibitors reduce pain and
inflammation
– competitive inhibitors of dihydrofolate reductase are
used in chemotherapy for cancer
• may specifically target bacteria and viruses
– competitive inhibitors of the viral protease are used to
treat HIV
In vivo, enzyme activity affected by
• availability of substrates and cofactors
• product accumulation
• rates of synthesis and degradation
• specific activation/ inactivation
– eg. by phosphorylation, often in response to hormonal
signals
– may shift balance between catabolism and anabolism
Regulatory enzymes
• regulation of key enzymes is crucial to the regulation
of cellular metabolism
• key enzymes = pacesetter enzymes
– set pace for entire reaction sequence
– usually catalyse the slowest step (the rate limiting
step) in the sequence
– often the first committed step
• adjust flows in response to substrate availability
and/or demand for products
Allosteric enzymes
• multi-subunit enzymes
• multiple ligand binding sites
• binding of first substrate molecule creates
conformational changes which affects binding of
subsequent substrate molecules
• binding of ligands other than substrate, at sites other
than the active site, may affect enzyme activity
Allosteric enzymes may also be
regulated by covalent modification
• Phosphorylation
determines whether
enzyme is active
– may occur in response to
hormonal signals
• Allosteric effectors
control active enzyme
Review Qs .... Part 2
2. Briefly define or describe
a. Km
b. Competitive inhibition
c. Allosteric enzyme
Are these terms relevant to all enzymes?
3. List four factors that could affect the activity of
an enzyme in vivo